(In reply to Benjamin Smedberg [:bsmedberg] from comment #3)
> What is the FB desktop plugin? Is it actually a browser plugin?
Apparently it comes with Facebook Desktop, an out-of-browser desktop integration of Facebook chat notifications etc.
The plugin is npfbdesktopplugin.dll, probably providing the connection with the desktop integration.

That plugin is part of Facebook Messenger for Windows. "Facebook Desktop" isn't an official name, but it's the same thing. Facebook Messenger for Android and iOS are unrelated products (slightly confusing), but the desktop app also pushed out a new version starting on March 4. This could be related, and I'm going to try to repro.
-- Jack O'Connor, Facebook Desktop team

I'm not able to repro the hang myself on Win7 or Win8 in Firefox 19. I see that the bug report is on Windows XP, so I'm going to try to get an XP machine to test. If anyone else wants to try to repro, you can download the desktop app here: https://www.facebook.com/about/messenger
To confirm that the plugin is working in any given page, you can run the following JS snippet and look for it to return non-null:
require("FBDesktopPlugin").isAppRunning()
The reporter mentioned that he was able to load Facebook in Chrome, but Chrome uses the same Netscape Plugin API that Firefox does, so I'm not sure how to explain that.

Successful repro in the "XP Mode" VM under Win7. Firefox hangs as reported, Chrome shows a plugin crashed butterbar, and IE 8 just fails to load the plugin at all.
The source of the crash looks like a slightly embarrassing build configuration issue that only affects Windows XP: http://blogs.msdn.com/b/vcblog/archive/2012/10/08/10357555.aspx I have a fix ready, and I will push it out as soon as I can.

Jack, excellent! Could somebody (in your QA team or ours) spend a minute figuring out why our plugin hang reporter isn't triggering in this case? It's probably sufficient to hang Firefox and then run this tool http://benjamin.smedbergs.us/crashfirefox.exe to actually force Firefox to crash and submit a crash report.

I had been seeing some "stop plugin?" dialogs before, which might've come after 45 seconds. When I tried the failing plugin again just now in FF19 on XP, I found that Facebook would totally hang the browser for 45 seconds, followed by maybe a second of UI updating/scrolling/whatever. That repeated at least three times and didn't show any signs of stopping. If your QA team wants to test the broken plugin, you can try the MSI in comment 11.

I was able to reproduce this issue as suggested, with the MSI from comment 11 and both Firefox 19.0 and Firefox 19.0.2. Firefox did indeed hang, and the Unresponsive Script dialog appeared, but I didn't get any crash.
Couldn't reproduce with the latest Nightly, build ID: 20130312031046.
Couldn't reproduce with the latest Aurora, build ID: 20130312042013.
Couldn't reproduce with the latest Beta, Firefox 20 beta 4, build ID: 20130307075451.